Science fiction loves to play off the potential threat of threat of alien viruses. But a new study suggests that space travellers are much more likely to be threatened by germs from our own planet that become more virulent in space.

Warding off infections is a real priority for astronauts, especially if longer space missions to the Moon and Mars are to go ahead. People have a tendency to get sick in space and over half of the astronauts on the Apollo missions became ill during their trips or soon after their return to Earth.

Earlier research has shown that prolonged weightlessness weakens our immune systems by preventing key sets of genes from switching on. But that’s only